Reformers for catalytically oxidizing hydrocarbons to produce hydrogen and carbon monoxide fuels are well known. Such reformers are used as fuel generators for downstream fuel cell systems in known fashion. Catalytic reforming requires an elevated catalyst temperature that at steady-state is typically between about 650° C. and 800° C. The reforming temperature then is maintained either by exothermic reforming or by endothermic reforming in the presence of hot exhaust recycled from the fuel cell system.
At start-up from an ambient temperature, the catalyst must be heated to a minimum temperature of about 500° C. before reforming can begin. One method for rapidly heating the catalyst is to combust oxygen and hydrocarbon fuel in an inline combustor ahead of the reformer and to pass the combustor exhaust through the reformer and then past the fuel cell anodes. In this practice, the combustor is operated optimally at a fuel-lean fuel:air ratio, whereas reforming is operated optimally at a very fuel-rich condition. Thus, it becomes of great importance to know when the catalyst surface reaches a temperature sufficient to support catalysis, in order to change over the mixture from combustion to reforming. If the changeover is too early, the catalyst temperature will be too low, and non-reformed hydrocarbons will be passed to the anodes, causing coking of the anodes and efficiency loss of the fuel cell system. If the changeover is too late, the reformer catalyst durability will be negatively impacted and the potential for pre-ignition in the reformer will be increased.
Obviously, a temperature probe at the catalyst surface could indicate when a suitable surface temperature has been reached. However, in practice such a location is not especially robust or practical and can also interfere with proper flow of gases through the reformer. Instead, a temperature probe typically is disposed within the ceramic elements of the reformer, which serves to protect the probe but also insulates it significantly, creating serious hysteresis between actual surface temperatures and measured temperatures during periods of rapid temperature change in the reformer.
One approach to dealing with this problem is to simply determine empirically how long it takes for the surface to reach the required minimum reforming temperature, and is then program the system controller to change the mixture after that time period. However, the length of time will depend upon the thermal state of the catalyst at start-up; the system may have been shut down only recently, in which case the reformer may still be quite warm, thus shortening the required combustion time. Indeed, if the reformer temperature is still sufficiently high to permit reforming, no combustion at all may be needed or desired. Also, the rate of heating will depend upon the latent combustive heat value of the fuel source being used, as well as the heat capacity and mass of the catalyst. Thus, neither a simple time instruction nor catalyst internal temperature measurement is adequate to determine when to change the entering mixture from combustion to reforming.
What is needed in the art is an improved means of estimating when to terminate combustion and change over to reforming.
It is a principal object of the present invention to change over a hydrocarbon reformer from combustion to reforming when the surface temperature of the catalyst exceeds a predetermined value.